This application pertains to the art of electrical amplifiers, and more particularly, to a current amplifier with a wide dynamic range and automatic drift correction. The invention is particularly applicable to the amplification of current signals from silicon photodiodes and more specifically, to scintillation crystal photodiode radiation detectors in conjunction with computerized tomographic scanning apparatus. The invention will be described with particular reference to computerized tomography. However, it will be appreciated that the invention has much broader applications such as, electrical meters, measuring devices, and other electrical apparatus that implement current amplification.
Generally, a computerized axial tomographic scanner comprises a source of radiation for irradiating a patient positioned within the scan circle and a plurality of radiation detectors positioned opposite the scan circle from the source of radiation. The origin of the source of radiation is caused to move about the scan circle to irradiate the patient from a plurality of directions. The detectors are positioned and monitored to determine the amount of radiation attenuation along a plurality of known paths crossing the scan circle. With well known computer reconstruction techniques, the radiation attenuation measured along the known paths is reconstructed into an image of the planar region of the patient. The radiation detectors generally comprise a scintillation crystal positioned to recieve the incident radiation and a photomultiplier tube optically coupled with the scintillation crystal. Alternately, ionized gas radiation detectors may be used.
However, photomultiplier tubes are costly, may have a poor time response, lack linearity and reliability, and draw relatively large amounts of power.
Photodiodes have found little acceptance as a replacement for photomultiplier tubes because they produce signals of much lower amplitudes. The low amplitudes create problems with noise and tend to increase the usable dynamic range.
The present invention contemplates a new and improved current amplifier which overcomes all of the above problems and others and provides an amplifier which is low in cost, has a wide, linear dynamic range and a uniforma response.